1. Field of the Invention
The present invention relates to an image forming apparatus such as a copying apparatus, a facsimile apparatus or a printer, capable of two-sided image formation.
2. Related Background Art
In such conventional image forming apparatus, a sheet is fed from a feeding tray by a pickup roller and through paired separating rollers and through a conveying path, further conveyed by paired registration rollers in synchronism with the rotation of a photosensitive drum, and receives, on an upper surface of the sheet, a toner image from the photosensitive drum in transfer means. Then the sheet is passed by a conveying unit and the toner image on the sheet is thermally fixed in fixing means.
After a photosensitive drum is primarily charged by charging means, the photosensitive drum is exposed by exposure means so that An electrostatic latent image is formed on a surface of the photosensitive drum. The electrostatic latent image is visualized with toner supplied by developing means so that a toner image to formed.
A configuration for executing the foregoing steps up to the thermal fixation of the toner image on the sheet constitutes image forming means. The image forming means may also have other configurations such as based on the ink jet system.
The sheet afterimage formation is then subjected to either straight discharge, or surface reversed discharge, or conveyed again to the transfer means for the purpose of two-sided image formation.
A two-side reversing unit is provided with paths for two-sided image formation, and is separable from the main body of the apparatus in a pullout type.
In the following, the conveying operation of the sheet after the fixing means will be explained in detail, following the flow of the sheet.
In case of two-sided image formation, the sheet is guided through a conveying path by the switching of a first discharge flapper, and guided to a surface reverse conveying path by the switching of a first duplex flapper.
The arrival of the conveyed sheet is detected by sheet detection means.
Based on a sheet arrival signal and information on the sheet length in the conveying direction, a CPU in the main body of the apparatus judges the timing of stopping and reversing (in the counterclockwise direction) of a roller and controls the driving of the roller.
The CPU stops and reverses the roller when the trailing end of the sheet passes a second duplex flapper but is positioned in front of a driven roller.
The conveyed sheet is conveyed by paired conveying rollers arranged in succession to the transfer means for the image formation on the second surface.
The image forming apparatus capable of executing the above-described operations has recently been digitized for example in the copying apparatus and is formed as a composite apparatus having also functions of printer and facsimile. For such composite apparatus, a higher speed is required in recent years in order to improve the productivity.
As a method for realizing such high speed, high speed conveying control is adopted for the sheet in the conveying path to the transfer means when the sheet is conveyed to the transfer means.
The sheet is conveyed from a sheet containing portion by a pickup roller to paired separating rollers, which conveys the uppermost one, among the sheets picked up by the pickup roller, to paired conveying rollers.
A sensor for detecting a leading end of the sheet is provided just before or just after the paired conveying roller. When the sensor detects the leading end of the sheet, the sheet is once stopped and waiting in a state that the sheet is nipped by the paired conveying rollers.
Thereafter, the conveying operation is started again at a timing capable of forming a predetermined distance to the immediately preceding sheet.
This is to enable an increase in the conveying speed of the sheet, at the high speed conveying control, by maintaining an appropriate distance to the immediately preceding conveyed sheet. If without such control, the immediately preceding sheet S conveyed at the predetermined speed by the paired registration rollers toward the transfer means may be caught up by the succeeding sheet S conveyed at the increased speed thereby resulting a damage in the sheet or a double conveying of the sheets.
After such correction of the conveying distance, the sheet is conveyed to the paired registration rollers at a speed higher than the conveying speed on the transfer means. Subsequently, this control is repeatedly executed.
Also, in case of two-sided image formation, for the purpose of high-speed conveying control, the leading end of the sheet is detected by a sensor provided in the middle of the re-conveying path, whereby the sheet is once stopped and waits in a predetermined position while it is pinched between the paired conveying rollers, and the sheet conveying is started again at a time when a predetermined distance is formed to the immediately preceding sheet.
The above-described conventional technology has however been associated with the following drawback.
In the conventional technology, it is necessary to secure, within the conveying path, a waiting space corresponding to the length of the sheet in the conveying direction, so that the conveying path has to be made sufficiently long.
Because of such long conveying path, the compactization or the space saving in the main body of the image forming apparatus has not been possible.
In consideration of the foregoing, the object of the present invention is to provide an image forming apparatus capable of compactization of the apparatus while maintaining a high productivity in the two-sided image formation.
The above-mentioned object can be attained, according to the present invention, by an image forming apparatus comprising:
image forming means for forming an image on a surface of a conveyed sheet;
a re-conveying path for re-conveying a sheet, on which an image has been formed on one side of the sheet, to the image forming means to form images on both sides of the sheet by the image forming means; and
a sheet containing portion for feeding the sheet to a conveying path interflowing to the middle of the re-conveying path;
wherein, in case the sheet, on which the image has been formed on one side of the sheet, is once stopped and waits in the re-conveying path, the leading end of the stopped and waiting sheet, on which the image has been formed on one side of the sheet is positioned downstream of the interflowing position.
In addition there are preferably provided:
a surface reverse conveying path provided upstream of the re-conveying path, for reversing the surfaces of the sheet on which the image has been formed on one side of the sheet;
an arc-shaped conveying path connecting the surface reverse conveying path and the re-conveying path; and
a conveying roller, having an external periphery at the surface on the inner diameter side of the arc-shaped conveying path.
In the present invention, as explained in the foregoing, in case a sheet on which an image has been formed on one side of the sheet is once stopped and waits in the re-conveying path, the leading end of the sheet on which the image has been formed on one side of the sheet, thus made to stop and wait, is positioned downstream of the interflowing position in the middle of the re-conveying path Where a conveying path from the sheet containing portion interflow, whereby the re-conveying path downstream of the interflowing position is utilized in common by the sheet newly fed from the sheet containing portion and the sheet made to stop and wait in the high speed conveying control. Consequently the waiting space for such stopped and waiting sheet, corresponding to the sheet length in the conveying direction, may be secured in the re-conveying path, whereby achieved are compactization and space saving of the main body of the image forming apparatus. It is thus rendered possible to achieve compactization of the apparatus, while attaining a high productivity by executing the high speed conveying control in the two-sided image formation.
Also because of the presence of a surface reverse conveying path provided upstream of the re-conveying path, for reversing the surfaces of the sheet on which the image has been formed on one side of the sheet, an arc-shaped conveying path connecting the surface reverse conveying path and the re-conveying path; and a conveying roller having an external periphery at the surface on the inner diameter side of the arc-shaped conveying path, the surfaces of the sheet on which the image has been formed on one side of the sheet is reversed before being conveyed to the re-conveying path, and there can be prevented a loss in time, caused by reversing the surfaces of the sheet during the subsequent high-speed conveying control. Also as the leading end of the sheet made to once stop and wait is positioned downstream of the interflowing position, the sheet once stopped and waited in the re-conveying path can be prevented from being interfered by the conveying rollers provided upstream of the re-conveying path.